Electron velocity modulation apparatus



July 5, 1960 D. C. ROGERS ELECTRON VELOCITY MODULATION APPARATUS Filed May 12. 1955 lnventar aver/#6 D. CROGERS 5 B MM A Home y ELECTRON VELOCITY MODULATION APPARATUS Douglas Cecil Rogers, Aldwych, London, England, as-

signor to International Standard Electric Corporation, New York, N.Y., a corporation of Delaware Filed May 12, 1955, Ser. No. 507,960

Claims priority, application Great Britain Aug. 5, 1954 -4 Claims. (Cl. BIS-3.5) V

The present invention relates to travelling wave tubes in which an electron beam interacts with one of the spatial harmonics of an electromagnetic wave guided along a slow-wave structure, such as a helix.

A spatial harmonic travelling wave tube amplifier has been described by S. Millman in Bell Laboratories Record for January 1951 at page 14 and a backward wave oscillator utilizing similar principles has been described in the August 1953 issue of the same journal by R. Kompfuer. In the tubes discussed in those two articles the slow wave structure has a periodic arrangement of gaps in which the electron beam is able to interact with the electromagnetic field only at these gaps. The propagation velocity of the fundamental mode of oscillation of the electromagnetic waves guided by the structure is very much greater than that of the beam electrons, the latter having velocities such that the difference between their time of flight and the propagation of the wave between adjacent gaps is. approximately that of an integral multiple of the oscillation period of the said fundamental mode. Thus an electron reacts at one gap with a wave and at the next gap with a succeeding wave in the same phase relationship. Interaction can evidently occur with an electron beam travelling in the same or in the opposite direction to the direction of propagation of the electromagnetic waves in the slow-wave structure. In the former case amplification can occur, and in the latter, by feedback through the slow-wave structure, a backward wave oscillator is achieved. H

A structure having discrete periodic gaps is not the only type of slow-wave structure adaptable to spatial harmonic interaction with an electron beam. Certain kinds of helices such as are used in conventional helixtype travelling wave tubes can also be used, as appears, for example, from the paper The Helix as a Backward Wave Circuit Structure, by D. A. Watkins and E. A. Ash, published in the Journal of Applied Physics for June 1954 at page 782. In particular, if one considers a conducting tape coiled into a helix then, to an electron travelling close to the ribbon, parallel to the axis of the helix, the structure will appear very similar to the periodic structures discussed above, interaction occurring at the gaps between adjacent turns.

In a spatial harmonic travelling wave tube, which expression is hereafter used to identify tubes constructed for interaction between an electron beam and one of the spatial harmonics of an associated slow-wave structure, the slow-wave structure is terminated at either end so as to reduce, as much as possible, end reflections over the range of frequencies for which the tube is designed to operate. For many applications, such as a voltage tuned oscillator, the spatial harmonic travelling wave tube may operate over a range of voltages including that appropriate to the amplification, by interaction with the fundamental mode of propagation, as in the conventional travelling-wave tube, of much lower frequency waves. The terminations of the slow-wave structure will usually be far from reflectionless at frequencies far below the Patented July 5, 1960 designed operating range so that oscillation at such a lower frequency may easily result.

In accordance with the present invention there is provided a spatial harmonic travelling wave tube comprising a slow-wave structure, such as a helix, which generates an electromagnetic field both within and externally of the structure having, at the desired frequency of operation, an axial phase velocity much greater than the velocity of electrons of an electron beam flowing within the structure parallel to the axis thereof, the beam interacting with spatial harmonics of the said field, characterized in this, that wave attenuation means is positioned at a distance from the said structure such as not substantially to afiect the growth of waves of the desired spatial harmonic mode but to suppress the growth of waves of a fundamental mode of lower frequency having an axial phase velocity approximately the same as the velocity of the beam electrons.

For an ideal arrangement of slow wave structure it is possible to specify the precise location of the attenuation means, but, in practice, due to the presence of support rods or other dielectric substances necessarily placed about the slow wave structure, the calculation of the external field can be of use only as a guide; recourse must be had to empirical methods of determining the required position.

To avoid confusion, reference is here made to British Patent No. 660,793 (G. C. Dewey-L. D. Smullin 2-7), which is concerned with placing attenuating material around the helix of a conventional kind of travelling wave amplifier so as to suppress oscillation due to the fast mode of the field set up by the helix. As will be shown later, the spacing of the attenuation means of the present invention and of the prior invention from the surface of the slow-wave structure differ by about 2:1 and their actions are quite distinct.

The present invention will be further described With reference to the accompanying drawing which illustrates its application to a backward wave oscillator employing a ribon-type helix.

In the embodiment illustrated in the drawing, the oscillator uses a special type of travelling wave tube 1 which comprises an electron gun 2 mounted at one end of a glass envelope 3, a metal tape coiled into a helix 4-, and an electron collector electrode 5 at the other end of the envelope 1. The tube is mounted on the axis of beam focussing coils 6 and 7 so as to project through a rectangular waveguide 8, the arrangement being similar to that of a conventional travelling wave tube except that there is no input waveguide and the output waveguide is situated adjacent the electron gun in the position usually occupied by the input waveguide of a travelling wave amplifier. The waveguide 8 and the helix 4 are coupled by the conventional arrangements of a probe antenna 9 and waveguide choke members 10 and 111. The helix is supported on rods 12 of insulating material and is terminated at the collector end of the tube by coating the support rods 12 near their ends with resistance material as indicated at 13 and also provide a lossy anchorage 14 for the end of the helix.

As has been explained above, an electron travelling parallel to the axis of the helix and close to the surface of the metal tape will encounter electromagnetic field conditions similar to those existing in a periodically gapped slow-wave structure, and will interact with the field only at the gaps between adjacent turns of the helix. Near the axis the field contributions of different circumferential regions of the helix will cancel one another, so that, so far as spatial harmonic operation is concerned, there is no eifective field on the axis. For this reason an annular electron beam is used and the electron gun comprises an annular cathode 15. In the drawing the Ar will be 0.355 of that at the helix surface.

gun is shown also to have a cup-shaped focussing electrode 16, a centrally apertured first anode 17 and, a second anode 18, the latter being formed on the end of a tubular rearward extension 19 of the waveguide choke member 11. 1

In order to establish a strong space-harmonic field, the potential difference between adjacent turns of the helix is made as large as possible. The length of the circumference of one turn is, therefore, made about half a wave length for the desired mid-bank frequency of operation. In the embodiment illustrated, for a midband frequency of 3000 megacycles per second and an electron beam voltage of 2500 at mid-band, the length of one turn of the helix 4 is made 5 cm. and the helix pitch is 5 mm. By varying the beam voltage over the range 1000 volts to 6000 volts, the oscillation frequency is changed continuously over a band of frequencies extending from about 2000 inc./s. to about 4000 mc./s. The helix dimensions having been chosen for maximum space harmonic interaction with the electron beam, it transpires that the same helix dimensions and a beam voltage of 2500 are just those providing good performance for a fundamental mode travelling wave amplifier at- 900 mc./s. Due to the inevitable mismatch of. the helix terminations at this frequency, far below the desired operating range, there is a considerable danger of self oscillation at or about 900 -me'./s. To avoid such self-oscillation, in accordance with the present invention, a film of resistive material, indicated in the drawing by the shading at 20, is placed about the helix at a distance therefrom where it will attenuate the fundamental mode around 900 mc. /s. without appreciably affecting the spatial harmonic fields in the band 2000 to 4000' mc./s. In the bodiments of the invention of British Patent No. 660,793,

the resistive film required for that invention for the suppression of fast 900 mc./s. modes conducted by a helix of the same form as used in the embodiment of the present invention would have to be spaced a distance of some 1.2 cms. from the surface of the helix about twice the distance required for use in the present invention.

By means of the present invention it is possible to employ, in a spatial harmonic travelling wave tube, a substantially aperiodic slow-wave structure which would also be suitable for use in a conventional travelling wave amplifier at a much lower frequency, and which, therefore, necessitates the provision of special measuresto avoid self-oscillation at thev lower frequency. The present invention provides a very simple solution to the difficulty, which would, otherwise, entail complicated extra matching arrangements for the helix terminations.

While the principles of the invention have been described above' in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and notas a limitation on the scope of the invention;

present embodiment the attenuation means takes the form of a resistive film placed on the envelope 3 of the oscillator tube, the dimensions of the latter being suitable for the purpose. In other embodiments,.wherethe envelope might be of a different diameter, the attenuation means couldtake other forms.

For example, the helix could he supported'inside the main envelope in a closely fitting glass tube, the exterior of which is coated with resistive material.

The approximate distance at which the attenuation A means should be spaced from the surface ofthe helix-can be determined as follows.

i It is known that the electromagnetic sen outside a helical sheet conductor decays as exp'( "yAr) where-Ar is the distance from the conducting surface and 'y is approximately equal to 27%., Ag being the guide wavelength, which for present purposes can be taken equal to A /lO, x being the free space wavelength. According to this invention the value 'yAr is preferably between 2 and 5. Now if the attenuation means be placed at a distance Ar such that Ar varies from 2.3 to 4.6 over the desired frequency range of 2000 mc./s. to 4000 mc./s., the field at distance Ar will always be less than of the field at the surface of the helix. However, at 900 mc./s. 'yAr will be 1.035 and the field at distance Hence, at 900 mc./s. strong attenuation will be presented and will prevent the growth of fundamental mode waves, While, for the space harmonic mode the attenuation means is too far removed from the helix to have any appreciable elfect. For the particular helix dimensions given in the embodiment described with reference to the drawing, the value of Ar in accordance with the above that in' practice the calculated distance can be taken only i as a guide and that the distance actually chosen must be determined from direct attenuation measurements,

termination at the other endof the helix, and a film of resistive material positioned about the said helix at a distance Ar from the surface of the slow-wave structure such that 'yAr lies in the range 2.0 to 5.0 at the desired frequencies of operation of the tube, where v is equal to Z'Ir/X k being the wavelength of the mid-band frequency of the desired band of operation to prevent growthof a forward travelling wave at a lower frequency than the desired oscillations while not substantially attenuating the saiddesired oscillations, said film extend- 'ing along the greater part of the axial length of said helix. 2. A backward wave oscillator tube comprising an evacuated envelope, an electron gun at one end. of the said envelope and an electron collector electrode at the other, a metal tape wound into a helix such that the circumferential length of each turn is of the order of one half the wavelength A of the desired oscillations measured along the axis of the said helix, waveguide-to.- helix coupling means at the electron gun end of the helix and non-reflecting termination means at the other end of the helix, and a film of resistive material placed at a distance Ar from the surface of the helix external to the same such that 'yAr lies between 2.0 and 5.0, 7 being equal to Z-n-A said film extending along the greater part of the axial length of said helix.

3. A backward wave oscillator according. to claim 2 in which the said film of resistive material is positioned externally of said envelope, the value of Ar including as a function the effect of dielectric material between the filth and the helix. 7 v

' 4. A spacial harmonic travelling wave tube comprising an electron beam source, a slow wave structure which generates for a desired spacial frequency mode an electromagnetic field at the desired operating'frequencyof said tube, having a phase velocity much greater thanthe velocity of the electrons of said beam, and also develops another electrpmag'netic field at a fundamental frequency and termination means for producing a substantial irnpedance match at one end of said structure for waves of said desired frequency characterized by additional wave attenuating means spaced radially of said structure at distance Ar from the surface of the slow-wave structure that 'yAr lies in the range 2.0 to 5.0 at the desired frequencies of operation of the tube, wher 'y is equal to 21r/A a being the wavelength of the mid-band frequency of the desired band of operation to substantially suppress said fundamental frequency waves but not to substantially affect the growth of waves of the spacial harmonic mode.

References Cited in the file 01 this patent UNITED STATES PATENTS Re. 24,460 Smullin et a1. Apr. 15, 1958 6 Touraton et a1. Sept. 9, 1952 Diemer Feb. 16, 1954 Lerbs Feb. 15, 1955 Lindenblad Mar. 5, 1957 Lerbs Sept. 24, 1957 Dallons Oct. 1, 1957 Robertson Jan. 14, 1958 Klein et a1. July 15, 1958 Klein et a1 Oct. 21, 1958 Ashkin Jan. 27, 1959 Webber et a1. Aug. 18, 1959 FOREIGN PATENTS Great Britain Nov. 18, 1953 

